1. Field of the Invention
This invention is in the field of fixed and moving target detection in Synthetic Aperture Radar (SAR) imaging.
2. Description of the Related Art
Synthetic Aperture Radar (SAR) radar is used for ground mapping as well as moving/stationary target identification. The general principle behind SAR is to coherently combine the amplitude and phase information of radar returns from a plurality of sequentially transmitted pulses. These pulses are from a relatively small antenna on a moving platform. As the platform moves, the information contained in the pulses is combined to arrive at a high resolution SAR image.
The plurality of returns creating a SAR image generated by the transmitted pulses along a presumed known path of the platform make up a frame length. Theoretically, during the frame length, amplitude as well as phase information returned from each of the pulses, for each of many range bins, is preserved. The SAR image is formed from the coherent combination of the amplitude and phase of return(s) within each range bin, motion compensated for spatial displacement of the moving platform during the acquisition of the returns for the duration of the frame length.
The plurality of pulses transmitted during an SAR frame length, when coherently combined and processed, result in image quality comparable to a longer antenna, corresponding approximately to the “length” traveled by the antenna during the frame length. The clarity of a SAR image is in many respects dependent on the quality of the motion compensation applied to each radar return prior to SAR image computation. Motion compensation shifts the phase of each radar sample (typically an I+jQ complex quantity derived from an analog to digital converter) in accordance with the motion in space of the moving platform. The SAR imaging process depends on the coherent, phase accurate summing of all radar returns expected within a frame.
The cost of airborne radars is dependent on the signal processing resources installed. Where a small antenna is used to allow for reduced cost, the clutter associated with the image is spread out in Doppler thus placing ground moving targets traveling at speeds of interest within the main lobe clutter. When coupled with relatively short array times necessitated to cover a large search area, a reduced cost radar is precluded from using multiple modes and waveforms to detect fixed and moving targets within the SAR image.